msgsnd (2) - Linux Manuals
msgsnd: System V message queue operations
Command to display msgsnd manual in Linux: $ man 2 msgsnd
NAME
msgrcv, msgsnd - System V message queue operations
SYNOPSIS
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/msg.h>
int msgsnd(int msqid, const void *msgp, size_t msgsz, int msgflg);
ssize_t msgrcv(int msqid, void *msgp, size_t msgsz, long msgtyp,
int msgflg);
DESCRIPTION
The
msgsnd()
and
msgrcv()
system calls are used to send messages to,
and receive messages from, a System V message queue.
The calling process must have write permission on the message queue
in order to send a message, and read permission to receive a message.
The
msgp
argument is a pointer to a caller-defined structure
of the following general form:
struct msgbuf {
long mtype; /* message type, must be > 0 */
char mtext[1]; /* message data */
};
The
mtext
field is an array (or other structure) whose size is specified by
msgsz,
a nonnegative integer value.
Messages of zero length (i.e., no
mtext
field) are permitted.
The
mtype
field must have a strictly positive integer value.
This value can be
used by the receiving process for message selection
(see the description of
msgrcv()
below).
msgsnd()
The
msgsnd()
system call appends a copy of the message pointed to by
msgp
to the message queue whose identifier is specified
by
msqid.
If sufficient space is available in the queue,
msgsnd()
succeeds immediately.
The queue capacity is governed by the
msg_qbytes
field in the associated data structure for the message queue.
During queue creation this field is initialized to
MSGMNB
bytes, but this limit can be modified using
msgctl(2).
A message queue is considered to be full if either of the following
conditions is true:
- •
-
Adding a new message to the queue would cause the total number of bytes
in the queue to exceed the queue's maximum size (the
msg_qbytes
field).
- •
-
Adding another message to the queue would cause the total number of messages
in the queue to exceed the queue's maximum size (the
msg_qbytes
field).
This check is necessary to prevent an unlimited number of zero-length
messages being placed on the queue.
Although such messages contain no data,
they nevertheless consume (locked) kernel memory.
If insufficient space is available in the queue, then the default
behavior of
msgsnd()
is to block until space becomes available.
If
IPC_NOWAIT
is specified in
msgflg,
then the call instead fails with the error
EAGAIN.
A blocked
msgsnd()
call may also fail if:
- •
-
the queue is removed,
in which case the system call fails with
errno
set to
EIDRM;
or
- •
-
a signal is caught, in which case the system call fails
with
errno
set to
EINTR;see
signal(7).
(msgsnd()
is never automatically restarted after being interrupted by a
signal handler, regardless of the setting of the
SA_RESTART
flag when establishing a signal handler.)
Upon successful completion the message queue data structure is updated
as follows:
- •
-
msg_lspid
is set to the process ID of the calling process.
- •
-
msg_qnum
is incremented by 1.
- •
-
msg_stime
is set to the current time.
msgrcv()
The
msgrcv()
system call removes a message from the queue specified by
msqid
and places it in the buffer
pointed to by
msgp.
The argument
msgsz
specifies the maximum size in bytes for the member
mtext
of the structure pointed to by the
msgp
argument.
If the message text has length greater than
msgsz,
then the behavior depends on whether
MSG_NOERROR
is specified in
msgflg.
If
MSG_NOERROR
is specified, then
the message text will be truncated (and the truncated part will be
lost); if
MSG_NOERROR
is not specified, then
the message isn't removed from the queue and
the system call fails returning -1 with
errno
set to
E2BIG.
Unless
MSG_COPY
is specified in
msgflg
(see below),
the
msgtyp
argument specifies the type of message requested, as follows:
- •
-
If
msgtyp
is 0,
then the first message in the queue is read.
- •
-
If
msgtyp
is greater than 0,
then the first message in the queue of type
msgtyp
is read, unless
MSG_EXCEPT
was specified in
msgflg,
in which case
the first message in the queue of type not equal to
msgtyp
will be read.
- •
-
If
msgtyp
is less than 0,
then the first message in the queue with the lowest type less than or
equal to the absolute value of
msgtyp
will be read.
The
msgflg
argument is a bit mask constructed by ORing together zero or more
of the following flags:
- IPC_NOWAIT
-
Return immediately if no message of the requested type is in the queue.
The system call fails with
errno
set to
ENOMSG.
- MSG_COPY (since Linux 3.8)
-
Nondestructively fetch a copy of the message at the ordinal position
in the queue specified by
msgtyp
(messages are considered to be numbered starting at 0).
-
This flag must be specified in conjunction with
IPC_NOWAIT,
with the result that, if there is no message available at the given position,
the call fails immediately with the error
ENOMSG.
Because they alter the meaning of
msgtyp
in orthogonal ways,
MSG_COPY
and
MSG_EXCEPT
may not both be specified in
msgflg.
-
The
MSG_COPY
flag was added for the implementation of
the kernel checkpoint-restore facility and
is available only if the kernel was built with the
CONFIG_CHECKPOINT_RESTORE
option.
- MSG_EXCEPT
-
Used with
msgtyp
greater than 0
to read the first message in the queue with message type that differs
from
msgtyp.
- MSG_NOERROR
-
To truncate the message text if longer than
msgsz
bytes.
If no message of the requested type is available and
IPC_NOWAIT
isn't specified in
msgflg,
the calling process is blocked until one of the following conditions occurs:
- •
-
A message of the desired type is placed in the queue.
- •
-
The message queue is removed from the system.
In this case, the system call fails with
errno
set to
EIDRM.
- •
-
The calling process catches a signal.
In this case, the system call fails with
errno
set to
EINTR.
(msgrcv()
is never automatically restarted after being interrupted by a
signal handler, regardless of the setting of the
SA_RESTART
flag when establishing a signal handler.)
Upon successful completion the message queue data structure is updated
as follows:
-
msg_lrpid
is set to the process ID of the calling process.
-
msg_qnum
is decremented by 1.
-
msg_rtime
is set to the current time.
RETURN VALUE
On failure both functions return -1
with
errno
indicating the error,
otherwise
msgsnd()
returns 0
and
msgrcv()
returns the number of bytes actually copied into the
mtext
array.
ERRORS
When
msgsnd()
fails,
errno
will be set to one among the following values:
- EACCES
-
The calling process does not have write permission on the message queue,
and does not have the
CAP_IPC_OWNER
capability in the user namespace that governs its IPC namespace.
- EAGAIN
-
The message can't be sent due to the
msg_qbytes
limit for the queue and
IPC_NOWAIT
was specified in
msgflg.
- EFAULT
-
The address pointed to by
msgp
isn't accessible.
- EIDRM
-
The message queue was removed.
- EINTR
-
Sleeping on a full message queue condition, the process caught a signal.
- EINVAL
-
Invalid
msqid
value, or nonpositive
mtype
value, or
invalid
msgsz
value (less than 0 or greater than the system value
MSGMAX).
- ENOMEM
-
The system does not have enough memory to make a copy of the
message pointed to by
msgp.
When
msgrcv()
fails,
errno
will be set to one among the following values:
- E2BIG
-
The message text length is greater than
msgsz
and
MSG_NOERROR
isn't specified in
msgflg.
- EACCES
-
The calling process does not have read permission on the message queue,
and does not have the
CAP_IPC_OWNER
capability in the user namespace that governs its IPC namespace.
- EFAULT
-
The address pointed to by
msgp
isn't accessible.
- EIDRM
-
While the process was sleeping to receive a message,
the message queue was removed.
- EINTR
-
While the process was sleeping to receive a message,
the process caught a signal; see
signal(7).
- EINVAL
-
msqid
was invalid, or
msgsz
was less than 0.
- EINVAL (since Linux 3.14)
-
msgflg
specified
MSG_COPY,
but not
IPC_NOWAIT.
- EINVAL (since Linux 3.14)
-
msgflg
specified both
MSG_COPY
and
MSG_EXCEPT.
- ENOMSG
-
IPC_NOWAIT
was specified in
msgflg
and no message of the requested type existed on the message queue.
- ENOMSG
-
IPC_NOWAIT
and
MSG_COPY
were specified in
msgflg
and the queue contains less than
msgtyp
messages.
- ENOSYS (since Linux 3.8)
-
Both
MSG_COPY
and
IPC_NOWAIT
were specified in
msgflg,
and this kernel was configured without
CONFIG_CHECKPOINT_RESTORE.
CONFORMING TO
POSIX.1-2001, POSIX.1-2008, SVr4.
The
MSG_EXCEPT
and
MSG_COPY
flags are Linux-specific;
their definitions can be obtained by defining the
_GNU_SOURCE
feature test macro.
NOTES
The inclusion of
<sys/types.h>
and
<sys/ipc.h>
isn't required on Linux or by any version of POSIX.
However,
some old implementations required the inclusion of these header files,
and the SVID also documented their inclusion.
Applications intended to be portable to such old systems may need
to include these header files.
The
msgp
argument is declared as struct msgbuf * in
glibc 2.0 and 2.1.
It is declared as void *
in glibc 2.2 and later, as required by SUSv2 and SUSv3.
The following limits on message queue resources affect the
msgsnd()
call:
- MSGMAX
-
Maximum size of a message text, in bytes (default value: 8192 bytes).
On Linux, this limit can be read and modified via
/proc/sys/kernel/msgmax.
- MSGMNB
-
Maximum number of bytes that can be held in a message queue
(default value: 16384 bytes).
On Linux, this limit can be read and modified via
/proc/sys/kernel/msgmnb.
A privileged process
(Linux: a process with the
CAP_SYS_RESOURCE
capability)
can increase the size of a message queue beyond
MSGMNB
using the
msgctl(2)
IPC_SET
operation.
The implementation has no intrinsic system-wide limits on the
number of message headers
(MSGTQL)
and the number of bytes in the message pool
(MSGPOOL).
BUGS
In Linux 3.13 and earlier,
if
msgrcv()
was called with the
MSG_COPY
flag, but without
IPC_NOWAIT,
and the message queue contained less than
msgtyp
messages, then the call would block until the next message is written
to the queue.
At that point, the call would return a copy of the message,
regardless
of whether that message was at the ordinal position
msgtyp.
This bug is fixed
in Linux 3.14.
Specifying both
MSG_COPY
and
MSC_EXCEPT
in
msgflg
is a logical error (since these flags impose different interpretations on
msgtyp).
In Linux 3.13 and earlier,
this error was not diagnosed by
msgrcv().
This bug is fixed
in Linux 3.14.
EXAMPLES
The program below demonstrates the use of
msgsnd()
and
msgrcv().
The example program is first run with the -s option to send a
message and then run again with the -r option to receive a
message.
The following shell session shows a sample run of the program:
$ ./a.out -s
sent: a message at Wed Mar 4 16:25:45 2015
$ ./a.out -r
message received: a message at Wed Mar 4 16:25:45 2015
Program source
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <time.h>
#include <unistd.h>
#include <errno.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/msg.h>
struct msgbuf {
long mtype;
char mtext[80];
};
static void
usage(char *prog_name, char *msg)
{
if (msg != NULL)
fputs(msg, stderr);
fprintf(stderr, "Usage: %s [options]\n", prog_name);
fprintf(stderr, "Options are:\n");
fprintf(stderr, "-s send message using msgsnd()\n");
fprintf(stderr, "-r read message using msgrcv()\n");
fprintf(stderr, "-t message type (default is 1)\n");
fprintf(stderr, "-k message queue key (default is 1234)\n");
exit(EXIT_FAILURE);
}
static void
send_msg(int qid, int msgtype)
{
struct msgbuf msg;
time_t t;
msg.mtype = msgtype;
time(&t);
snprintf(msg.mtext, sizeof(msg.mtext), "a message at %s",
ctime(&t));
if (msgsnd(qid, &msg, sizeof(msg.mtext),
IPC_NOWAIT) == -1) {
perror("msgsnd error");
exit(EXIT_FAILURE);
}
printf("sent: %s\n", msg.mtext);
}
static void
get_msg(int qid, int msgtype)
{
struct msgbuf msg;
if (msgrcv(qid, &msg, sizeof(msg.mtext), msgtype,
MSG_NOERROR | IPC_NOWAIT) == -1) {
if (errno != ENOMSG) {
perror("msgrcv");
exit(EXIT_FAILURE);
}
printf("No message available for msgrcv()\n");
} else
printf("message received: %s\n", msg.mtext);
}
int
main(int argc, char *argv[])
{
int qid, opt;
int mode = 0; /* 1 = send, 2 = receive */
int msgtype = 1;